Energy and Conventional and Advanced Exergy Analyses of Low-Temperature Geothermal Binary-Flashing Cycle Using Zeotropic Mixtures

Abstract

Due to its deep utilization of geobrine and its high net power output, the binary-flashing cycle (BFC) is deemed to be the future geothermal energy power generation technology. The working fluids considered in present analysis are zeotropic mixtures (R245/R600a). The system thermodynamic model is built, and the energy and conventional and advanced exergy analyses are carried out to reveal the real optimization potential. It is demonstrated that the optimal ranges of R245fa mass fraction and working fluid dryness at the evaporator outlet are 0.30~0.50 and 0.40~0.60, considering the thermodynamic performance and the flammability of the zeotropic mixtures, simultaneously. Conventional exergy analysis indicates that the maximum exergy destruction occurs in the condenser, followed by the expander, evaporator, flashing tank, preheater, high-pressure pump and low-pressure pump. Meanwhile, the advanced exergy analysis reveals that the expander should be given the first priority for optimization, followed by the condenser and evaporator. The BFC has a large potential for improvement due to higher avoidable exergy destruction, about 48.6% of the total system exergy destruction can be reduced. Moreover, the interconnections among system components are not very strong, owing to small exogenous exergy destructions. It also demonstrates the effectiveness of advanced exergy analysis, and the approach can be extended to other energy conversion systems to maximize the energy and exergy savings for sustainable development.

Keywords

• advanced exergy analysis
• geothermal energy
• binary-flashing cycle
• zeotropic mixtures
• inerting mass concentration